Laser array sidelobe suppression

ABSTRACT

A laser apparatus comprising an aperture module array including two or more aperture modules, each aperture module of the array being optically couple-able to a source of coherent electromagnetic radiation and configured to emit a beam of radiation received from the source. The apparatus emits a composite beam comprising beams emitted by the respective aperture modules and modulates at least one of the beams in power and phase relative to at least one other of the beams such that a desired non-uniform composite beam profile is provided.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND Field

This application relates generally to a laser array for directingcoherent electromagnetic radiation toward a target.

DESCRIPTION OF RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR1.97 AND 1.98

When a laser array, such as a high energy phased laser array, isdirected toward a target, a significant amount of the array'stransmitted radiant energy is diverted into beam sidelobes due todiffraction effects from finite spacing of individual array apertures. Asingle laser beam having certain transverse energy profiles (forexample, having a transverse energy profile corresponding to azero-order Bessel function) is known to resist diffraction better than abeam having a uniform, or even a Gaussian, transverse energy profile.However, known attempts to impart diffraction-resistant wavefrontprofiles to a composite beam emitted by a laser aperture array have beenlimited to geometric arrangement optimization of such aperture arrays.This solution mode is capable of demonstrating significant gains, butthe energy diverted to sidelobes is not decreased sufficiently to matchthe performance of a single monolithic laser beam of similar totaldiameter.

Laser arrays are also known to employ imaging systems that use digitalholographic imaging arrays to collect laser light reflected from atarget. Data obtained by such imaging systems is used to generate animage of a portion of a surface of the target, identify and correct forlaser distortions caused by aberrations and interference, and providephase information needed to control the phase of the transmitters in aphased laser array. Known imaging arrays use an array of receiveapertures that are positioned remotely from transmit apertures of thearray. Such remote placement can be inconvenient or impracticable inairborne and other space and/or weight-limited applications.

SUMMARY

A laser apparatus is provided comprising an aperture array including twoor more aperture modules, each aperture module of the array beingoptically couple-able to a source of coherent electromagnetic radiationand configured to emit a beam comprising radiation received from thesource. The apparatus is configured to emit a composite beam comprisingbeams emitted by two or more of the aperture modules and to modulate atleast one of the beams emitted by the two or more aperture modules inpower and phase relative to a beam emitted by at least one other of theaperture modules such that a desired non-uniform composite beam profileis provided that reduces the amount of energy distributed into sidelobesrelative to the amount of energy that would be distributed intosidelobes if the composite beam were of the same emitted energy but hada uniform composite beam exit profile.

Alternatively, the laser apparatus may comprise an aperture module arrayincluding one or more sources of coherent electromagnetic radiation, andtwo or more aperture modules that are optically coupled to and receivecoherent electromagnetic radiation from the one or more sources and emitrespective beams of the coherent electromagnetic radiation received fromthe one or more sources. A driving system may be connected to at leastone of the sources of coherent electromagnetic radiation and to at leasttwo of the aperture modules and may be configured to modulate at leastone of the beams emitted by the two or more aperture modules in powerand phase relative to a beam emitted by at least one other of the two ormore aperture modules such that a desired non-uniform composite beamprofile is provided. The driving system may also or alternatively beconfigured to steer the beams.

As a further alternative, the laser apparatus may comprise an aperturemodule array including three or more aperture modules, each aperturemodule of the array being optically couple-able to a source of coherentelectromagnetic radiation and configured to emit a beam comprising suchradiation. The apparatus may be configured provide a desired non-uniformcomposite beam profile by emitting a composite beam comprising beamsemitted by two or more of the aperture modules, modulating at least oneof the beams emitted by the two or more aperture modules in power andphase relative to a beam emitted by at least one other of the two ormore aperture modules, and causing at least one aperture module to emitno electromagnetic radiation.

DRAWING DESCRIPTIONS

These and other features and advantages will become apparent to thoseskilled in the art in connection with the following detailed descriptionand drawings of one or more embodiments of the invention, in which:

FIG. 1 is a front schematic view of a sidelobe-suppressing laserapparatus shown emitting a composite beam comprising coherent beams ofelectromagnetic radiation emitted from respective aperture modules of anaperture array, the differing intensities of electromagnetic radiationof each beam being represented in grayscale;

FIG. 2 is a side schematic cross-sectional view of the apparatus of FIG.1 taken along line 2-2 of FIG. 1 and including a composite beam profilerepresentation of the different intensities of electromagnetic radiationemitted by the respective aperture modules across the cross-section;

FIG. 3 is a side perspective view of the apparatus of FIG. 1 engaging atarget missile, and showing a stylized representation of the impingementof a composite beam on a target, with the transmitted beam having anapproximated Bessel beam profile;

FIG. 4 is a front schematic view of a second embodiment of asidelobe-suppressing laser apparatus with the apparatus shown emitting acomposite beam comprising coherent beams of varying intensities ofelectromagnetic radiation emitted from respective aperture moduleswithin meta-aperture modules of an aperture module array, the differentintensities of electromagnetic radiation being represented in grayscale;and

FIG. 5 is a partial side schematic cross-section view of the apparatusof FIG. 4 taken along line 5-5 of FIG. 4 and including a composite beamprofile representation of the different intensities of electromagneticradiation emitted by the respective aperture modules across thecross-section.

DETAILED DESCRIPTION

A sidelobe-suppressing laser apparatus is generally shown at 10 in FIGS.1-5. A second embodiment of the device is generally shown at 10′ inFIGS. 4 and 5. Reference numerals with the designation prime (′) inFIGS. 4 and 5 indicate alternative configurations of elements that alsoappear in the first embodiment. Unless indicated otherwise, where aportion of the following description uses a reference numeral to referto FIGS. 1-3, that portion of the description applies equally toelements designated by primed numerals in FIGS. 4 and 5.

The apparatus 10 may comprise an aperture module array 12 including twoor more aperture modules 14, each aperture module 14 of the array 12being optically couple-able to a source 16 of coherent electromagneticradiation (e.g., a laser) and configured to emit a beam 18 comprisingsuch radiation received from the source 16 as shown in FIG. 2. As shownin FIG. 1, the array 12 may include, for example, one hundredninety-nine aperture modules 14 arranged in a hexagonal grid. In otherembodiments the array 12 may include any number of aperture modules 14arranged in any suitable pattern.

The apparatus 10 is configured to emit a composite beam 20 and to directthe composite beam 20 toward a target 22. The composite beam 20comprises beams 18 emitted by the respective aperture modules 14. Atleast one of the beams 18 emitted by the respective aperture modules 14may be modulated in power and phase relative to a beam 18 emitted towardthe target 22 by at least one other of the aperture modules 14 such thata desired non-uniform composite beam profile 24 is provided.

In this description, the term “aperture module 14” means a moduleconfigured and positioned to receive electromagnetic radiation from asource 16 of coherent electromagnetic radiation and to emit thatelectromagnetic radiation in the form of a beam 18.

In this description, the term “composite beam profile 24” is intended asa collective reference to a pattern of intensities of respective beams18 that make up a composite beam 20 emitted by the apparatus 10.Accordingly, the term “non-uniform composite beam profile 24” isintended to refer to a composite beam intensity profile in which theintensities of the respective beams 18 of the composite beam 20 are notall equal to each other. Cross sections of representative composite beamprofiles are shown in FIGS. 2 and 5.

As shown in FIG. 2, the apparatus 10 may be configured to provide anon-uniform composite beam profile 24 that approximates a Besselfunction 26, thereby improving the resistance of the composite beam 20to diffraction and sidelobe energy loss. However, the apparatus 10 maybe configured to modulate the electromagnetic beams 18 emitted by theaperture modules 14 to provide any other desired non-uniform compositebeam profile 24.

These beam profile approximations may be improved by the employment ofimage enhancement techniques. A non-uniform composite beam profile 24permits aperture modules 14 to be assigned null transmission values,which are ideal for receiving and imaging. This addresses the need toperform aperture corrections from on-axis imagers within the array. Forexample, anti-aliasing techniques may be used to “soften” contoursprojected by the hexagonal matrix pattern of the aperture array 12 topresent an apparently smoother approximation of the more rounded Besselfunction 26.

As shown in FIG. 2, each aperture module 14 may include a steeringmodule 28 configured to direct electromagnetic radiation that passesthrough the aperture module 14. The steering module 28 may be used todirect or steer an emitted beam 18 of such electromagnetic radiationtoward a desired point on a target 22 (as shown in FIG. 3), or to aimtoward and receive radiation from a desired direction. The steeringmodule 28 may be of any suitable type including, for example, a RisleyIntegrated Steering Module such as is disclosed in U.S. Pat. No.7,898,712.

As shown in FIGS. 1 and 2, the apparatus 10 may be configured to providea desired non-uniform composite beam profile 24 by modulating the beams18 emitted by the aperture modules 14 such that the one or more of theaperture modules 14 emit electromagnetic radiation at a first level ofintensity while one or more other of the aperture modules 14 emitelectromagnetic radiation of a second lesser or greater level ofintensity. Alternatively, or in addition, the apparatus 10, may beconfigured to provide a desired non-uniform composite beam profile 24 bymodulating the electromagnetic output of the aperture modules 14 suchthat one or more of the aperture modules 14 emit electromagneticradiation at a first level of intensity while one or more other of theaperture modules 14 emit no electromagnetic radiation at all. In otherwords, many desired composite beam profiles may not require any outputat all from certain aperture modules 14 of the aperture module array 12.

As shown in FIG. 2, the apparatus 10 may include an imaging processor 30connected in communication with and configured to receive imaginginformation from the aperture modules 14. The imaging processor 30 maybe configured to determine target information by interpretinginformation received from certain aperture modules 14 of the array 12that are receiving electromagnetic radiation from the target 22 but arenot emitting electromagnetic radiation toward the target 22. Forexample, laser emissions from a composite beam 20 emitted by the array12 may reflect back from a target 22, with some of this reflectedradiation being received by the aperture modules 14. The aperturemodules 14 may then communicate information to the processor 30regarding the received radiation. The processor 30 may, in turn, beconfigured to calculate target information based on the informationreceived from aperture modules 14 that are receiving reflected radiationfrom the target 22 rather than emitting beams 18 at the target 22.Aperture modules 14 that are emitting beams 18 may not be usable by theprocessor 30 to calculate target information via received radiation, asthey may be saturated with backscatter from their emitted beams 18.However, aperture modules 14 that are emitting beams 18 may be usable bythe processor 30 to calculate target information if the receivedradiation is from a separate illuminator source and of a differentwavelength. The processor 30 may therefore be configured to useinformation received from non-emitting and/or emitting aperture modules14 to generate a digital holographic image of the surface of the target22.

As shown in FIG. 2, the apparatus 10 may include a driving system 32that may be connected to the source 16 (or sources) of coherentelectromagnetic radiation, and to one or more aperture modules 14. Thedriving system 32 may be configured to command the source(s) 16 and oneor more aperture modules 14 to modify their respective electromagneticoutputs. The imaging processor 30 may be connected to the driving system32 and configured to provide the driving system 32 with targetinformation. The driving system 32 may be configured to modify the beamsemitted by the aperture modules 14 in response to the target informationreceived from the imaging processor 30. The driving system 32 may, forexample, include a driving processor 34 that is in communication withthe electromagnetic radiation sources 16 and the steering modules 28,and that may be programmed or otherwise configured to command the source16 and the steering modules 28 to alter the direction, intensity, and/orphase of beams 18 comprising the composite beam 20 in response to targetinformation received from the imaging processor 30.

According to the second embodiment, and as shown in FIGS. 4 and 5, theapparatus 10′ may include a plurality of meta-aperture modules 36, eachsuch meta-aperture module 36 comprising a plurality of aperture modules14′ of the aperture module array 12′, and each meta-aperture module 36being configured to steer the beams emitted by its aperture modules 14′via meta-aperture steering modules 38. Each such meta-aperture steeringmodule 38 may be configured to collectively direct beams ofelectromagnetic radiation 18 that pass through the aperture modules 14′within a meta-aperture 36. According to this second embodiment, eachmeta-aperture steering module 38 may be connected to the driving system32′, and may comprise a Risley Integrated Steering Module, but otherembodiments may employ any other suitable type of meta-aperture modulesteering arrangement.

As shown in FIG. 4, the apparatus 10′ of the second embodiment mayinclude thirty-seven meta-aperture modules 36, and each may comprise acluster or sub-array of seven aperture modules 14′. As is also shown inFIG. 4, the meta-aperture modules 36 may be arranged in a hexagonalgrid, and the aperture modules 14′ may be arranged in hexagonal gridswithin each meta-aperture module.

In the second embodiment, each aperture steering module 28′ may comprisea steerable light fiber laser output device 40, as shown in FIG. 5. Thesteerable light fiber laser output device 40 may comprise a steeringmechanism 42 operably connected to a light fiber 44 such as afiber-optic cable that is couple-able to the coherent electromagneticradiation source 16. The steering mechanism 42 may be configured tomanipulate the orientation of an output end of the light fiber 44 to aimin a desired direction a laser beam 18 emitted from the output end ofthe light fiber 44. The steering mechanism 42 may comprise afine-steering Risley Integrated Steering Module, a piezo-activatedmicro-gimbal or actuator, an electronically-controlled liquid crystalsteering element, or any other suitable arrangement for steering laserbeams from fiber outputs.

In other words, beams emitted by an aperture module 14′ of ameta-aperture module 36 may be steered by its aperture steering module28′ and, either simultaneously or sequentially, by the meta-aperturesteering module 38 of the meta aperture module 36. Since any type ofsteering device may be assigned to either the aperture or meta-aperturesteering modules 28′, 38, different steering tasks may be divided amongthem. For example; in the second embodiment, the aperture steeringmodules 28′ may be used for fine steering adjustments, while themeta-aperture steering modules 38 may be used for coarse steeringadjustments.

An apparatus constructed as described above minimizes energy waste byemitting a sidelobe-suppressing composite beam. It can also observe theeffect and disposition of the composite beam upon a target withouthaving to remotely locate an array of receive apertures. This allows theapparatus to make adjustments to compensate for disruptions such asatmospheric distortions, target shape, and target motion, therebyproviding a more pronounced desired effect upon a target.

This description, rather than describing limitations of an invention,only illustrates embodiments of the invention recited in the claims. Thelanguage of this description is therefore exclusively descriptive and isnon-limiting. Obviously, it's possible to modify this invention fromwhat the description teaches. Within the scope of the claims, one maypractice the invention other than as described above.

What is claimed is:
 1. A laser apparatus comprising: an aperture modulearray including two or more aperture modules; each aperture module ofthe array being optically couple-able to a source of coherentelectromagnetic radiation and configured to emit a beam comprisingradiation received from the source; the apparatus being configured to:emit a composite beam comprising beams emitted by two or more of theaperture modules, and modulate in power and phase at least one of thebeams emitted by the two or more aperture modules relative to a beamemitted by at least one other of the two or more aperture modules suchthat a desired non-uniform composite beam exit profile is provided thatreduces the amount of energy distributed into sidelobes relative to theamount of energy that would be distributed into sidelobes if thecomposite beam were of the same emitted energy but had a uniformcomposite beam exit profile.
 2. A laser apparatus as defined in claim 1,further configured to modulate in power and phase at least one of thebeams emitted by the two or more aperture modules relative to a beamemitted by at least one other of the two or more aperture modules suchthat the resulting non-uniform exit profile provides increasedresistance to diffracting effects of phase noise and improved compositebeam shape retention over what a uniform composite beam exit profilewould provide at a given emitted energy level.
 3. A laser apparatus asdefined in claim 2 in which the apparatus is configured to provide adesired non-uniform composite beam profile resembling at least part of azero order Bessel function.
 4. A laser apparatus as defined in claim 1in which the apparatus is configured to provide a desired non-uniformcomposite beam profile by modulating the beam of at least one pluralityof aperture modules relative to at least one other plurality of aperturemodules.
 5. A laser apparatus as defined in claim 1 including anaperture steering module configured to direct electromagnetic radiationthat passes through at least one aperture module.
 6. A laser apparatusas defined in claim 5 in which the aperture steering module comprises aRisley Integrated Steering Module.
 7. A laser apparatus as defined inclaim 5 in which the aperture steering module comprises a fine-steeringRisley Integrated Steering Module.
 8. A laser apparatus as defined inclaim 5 in which the aperture steering module comprises apiezo-activated actuator.
 9. A laser apparatus as defined in claim 5 inwhich the aperture steering module comprises anelectronically-controlled liquid crystal steering element.
 10. A laserapparatus as defined in claim 1 including at least one meta-aperturemodule comprising two or more aperture modules of the aperture modulearray and configured to steer the beams of its two or more aperturemodules.
 11. A laser apparatus as defined in claim 10 including ameta-aperture steering module configured to direct electromagneticradiation that passes through the at least one meta-aperture module. 12.A laser apparatus as defined in claim 11 in which the meta-aperturesteering module is a Risley Integrated Steering Module.
 13. A laserapparatus as defined in claim 1, in which the apparatus is configured toprovide a desired non-uniform composite beam profile by causing at leastone aperture module to emit electromagnetic radiation while causing atleast one other aperture module to emit no electromagnetic radiation.14. A laser apparatus as defined in claim 13 including an imagingprocessor connected in communication with at least one aperture moduleand configured to determine target information by interpretinginformation received from at least one aperture module that is notemitting electromagnetic radiation.
 15. A laser apparatus as defined inclaim 14 in which: the apparatus includes a driving system connected toa source of coherent electromagnetic radiation of at least one aperturemodule and connected to at least one aperture module, and configured tomodify the electromagnetic output from the at least one aperture modulein response to target information; and the imaging processor isconnected to the driving system and is configured to provide targetinformation to the driving system.
 16. A laser apparatus as defined inclaim 15 in which the driving system is configured to modify theelectromagnetic output from at least one aperture module by altering, inresponse to the target information, at least one of the direction,intensity, or phase of a beam emitted by the at least one aperturemodule.
 17. A laser apparatus comprising an aperture module arrayincluding: one or more sources of coherent electromagnetic radiation;two or more aperture modules that are optically coupled to and receivecoherent electromagnetic radiation from the one or more sources and emitrespective beams of the coherent electromagnetic radiation received fromthe one or more sources; and a driving system that is connected to atleast one of the sources of coherent electromagnetic radiation and to atleast two of the aperture modules, is configured to modulate at leastone of the beams emitted by the two or more aperture modules in powerand phase relative to a beam emitted by at least one other of the two ormore aperture modules such that a desired non-uniform composite beamprofile is provided, and is further configured to steer the beams.
 18. Alaser apparatus as defined in claim 17 in which the apparatus isconfigured to provide a desired non-uniform composite beam profileresembling at least part of a zero order Bessel function.
 19. A laserapparatus comprising: an aperture module array including three or moreaperture modules; each aperture module of the array being opticallycouple-able to a source of coherent electromagnetic radiation andconfigured to emit a beam comprising radiation received from such asource; the apparatus being configured to provide a desired non-uniformcomposite beam profile by: emitting a composite beam comprising beamsemitted by two or more of the aperture modules, modulating at least oneof the beams emitted by the two or more aperture modules in power andphase relative to a beam emitted by at least one other of the two ormore aperture modules, and causing at least one aperture module to emitno electromagnetic radiation.
 20. A laser apparatus as defined in claim19 in which the apparatus is configured to provide a desired non-uniformcomposite beam profile resembling at least part of a zero order Besselfunction.